It is known to hydrogenate nitrobenzene and other nitroaromatics to yield the corresponding aromatic amines in the gas phase on immobilised supported catalysts. FR 25 25 591, for example, thus describes a process for hydrogenating nitrobenzene on stationary copper catalysts. DE-A 2 135 155 and 2 244 401 futhermore describe processes for reducing nitro compounds in the presence of supported catalysts containing palladium using spinels as the support materials. DE-A 2 849 002 also describes a process for the catalytic hydrogenation of nitrobenzene, in which hydrogenation is performed in the presence of a multi-component supported catalyst. A disadvantageous feature of the gas phase hydrogenations described in the stated patent publications is the low loading (specific loading) of the catalysts. The stated or calculated loadings vary between 0.2 and 0.6 kg/l.times.h. Loading is here defined as the quantity of nitroaromatics in kg which are reacted per litre of catalyst bed within one hour. The low catalyst loading is accompanied by an unsatisfactory space/time yield in large scale industrial processes for the production of aromatic amines.
DE-A 4 039 026 describes a gas phase hydrogenation on palladium catalysts which may be highly loaded. Catalyst loading in this process is between 0.6 and 0.95 kg/l.times.h. Industrial performance of the process has, however, revealed that after only a short time conversion of the nitrobenzene proceeds only incompletely, such that the condensate contains not inconsiderable quantities of unreacted nitrobenzene in addition to the aromatic amine which has been formed. If the nitrobenzene content is to be reduced to below the tolerable limit, the aniline must be purified by elaborate purification methods (distillation). It has furthermore been found that, at the high catalyst loading, the catalysts used in DE-A 4 039 026 have only an unsatisfactory service life.
The object of the present invention was to provide a process for the production of aromatic amines by catalytic hydrogenation of the corresponding nitro compounds in the gas phase, which may be performed without problems on an industrial scale and which ensures an elevated space/time yield, combined with improved productivity of the catalysts used.
This objective is achieved with the process according to the invention.